| ALWC
is a microbially influenced corrosion (MIC) relying
on a consortium of both aerobic (Sulphide Oxidising
Bacteria -SOB), and anaerobic (Sulphate Reducing Bacteria
-SRB).
There is a symbiotic relationship between the bacteria
and the electrochemical corrosion environment at the
steel face, which allows both to thrive at the expense
of the steel structure.
The ‘classic’ ALWC phenomenon is typified
initially by localised corrosion generally found within
a 0.5 metre band at Lowest Astronomical Tide (LAT) level
See image 1. The
corrosion is a pitting form of corrosion, suggesting
an intermittent or cyclical corrosion pattern, and can
be surprisingly localised See
image 2.
Forecasting the corrosion rate is therefore difficult
and is a major concern in the maintenance of existing
structures.
Average rates (calculated from installation to perforation)
of 0.3 to 0.5mm per year are most common, but rates
of 0.8 to 1.0mm per year are not uncommon. In certain
environments concentrated corrosion may show rates as
high as 4mm per year (SRB are known to corrode materials
at higher rates in ideal conditions). It is therefore
advisable to take specialist advice at an early stage
when planning a project.
Evidently, the life of a structure will fall into distinct
periods:
when steel thickness is at design optimum 'with
corrosion allowance often included'.
when there has been substantial steel loss but
within factor of safety.
And later when sections are in danger of being
holed, and ultimately are perforated.
Structures in any of these states, if ALWC has not been
designed against, will require active consideration by
the owner / maintainer, but ALWC preferentially corrodes
some parts of steel sections See
image 3, making condition surveying, interpretation
and forecasting difficult.
For more
detail of structures with age see:
The design advice, to meet also Construction Design
and Management Regulations (CDM), must be that concentrated
corrosion can be avoided and should be designed against,
or a planned maintenance strategy put in place, pre-construction.
For further comments on specific affects and advised
activity see:
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